1. Field of the Invention
The present invention relates to an apparatus for reproducing an image on a cathode-ray tube, including a video signal processing circuit for providing inter alia a preamplification of the video signal either with a given maximum gain or a reduced gain with respect to the maximum gain to modify the image contrast, said processing circuit having a contrast control terminal for receiving a variable control voltage, the apparatus also including a contrast control circuit comprising:
a device for comparing peak values of the video signal (of each video signal R (red), V {green) and B blue)), to a reference threshold, and having an output -or applying a signal which is supplied when this threshold is exceeded, after integration, to said contrast control terminal; and PA1 a manual contrast adjustment device for supplying a signal which is also applied to said contrast control terminal.
The contrast adjustment of a video image reproduced on a cathode-ray tube has for its object to satisfy several technical or esthetical requirements.
In the first place, it is current usage to limit the average value of the electron beam current of the tube, mainly to ensure an operating life of this tube in accordance with the life expectancies specified by its manufacturer (premature ageing effect of the cathodes when the mean beam current is too high), and also to ensure that the circuits which produce the very high voltage can correctly function with the utmost safety. For the case in which a very bright image must be reproduced, there is then every reason to reduce the amplification of the video signal while maintaining the black level, until a beam current is obtained which does not exceed the fixed limit. This actually means that an automatic reduction of the image contrast is produced, if this is necessary, so as not to exceed a given limit of the average beam current. These considerations also hold when a monochrome image reproduction or a color image reproduction is involved, in which case the notion average beam current must be understood to mean in actual practice the sum of the currents of the three electronic beams corresponding to the three guns of the color picture tube.
In the second place, and in accordance with more recent practice, also an additional automatic contrast reducing device is used which becomes operative when in certain images there are portions which are too bright for a correct reproduction, even if the ratio between the surface area of these portions is small compared with the overall surface of the image (and consequently do not in any significant manner influence the average beam current).
The cause of said error in the image reproduction mainly resides in the fact that during the reception of an excessively bright image portion, the corresponding signal causes a saturation of the video amplifier, which saturation, followed by a non-disregardable desaturation delay, produces an unacceptable halo in the image at the edges of the image portions. A further error associated with overbright image portions consist in a geometrical deformation of the remaining part of the image, which error is caused by the instantaneous overloading of the line scanning device. Thoughts might go towards effecting a clipping of the video signal peaks to limit the brightness of these exceptional image portions, but this would result in an unacceptable color of these image portions since this clipping would modify the respective portions of the signals R, G and B relative to the original proportions. In practice, one is compelled to act again on the contrast to bring the video signal level of the brightest portions of the image to a predetermined limit. Such an automatic gain reduction device is utilized for the case of monochrome or color reproduction.
In addition, measures are generally taken to provide the user with a manual contrast control so as to render it possible for him, for the same reason as for the brightness color adjustments, to adapt the reproduction to the exterior conditions (ambient brightness) and also to suit it to his personal taste.
2. Description of the Related Art
An example of automatic contrast control as a function of the ambient brightness, for a television set, is disclosed in the document U.S. Pat. No. 3,147,341 which, or that purpose, proposes to act on the automatic gain control circuit of the radio frequency and intermediate frequency (IF) stages. Actually, preference is however given to operate at a constant signal level at the output of the IF stage so as to ensure that the subsequent stages function in the optimum conditions for which these stages have been designed.
There appears to be one difficulty as regards the brightness adjustment arising from the fact that the multiple actions, automatic and manual, all end in a modification of the voltage of one single contrast control terminal of the video signal processing circuit. To the extent to which one wants to increase the efficiency of the automatic corrections, a reduction of the manual control effect is ultimately effected to the extent that in the majority of cases, there is a manual control range in which no visible effect is obtained, since the action on this control is compensated by the automatic limiter devices. To that effect, the brightness control button is even no longer included in the remote control housing, but is only placed on the television set itself.